I don't think Walmart or traditional grocers like Kroger will be hugely impacted by this. They still have way more stores than Whole Foods has, and they don't service the same customers anyway. Amazon would ruin the Whole Foods brand if it tries to change it into a pureplay competitor for Kroger, as that would mean replacing many niche expensive products to generic cheap items.
Amazon is going after a niche, high income, high margin market with Whole Foods. If they truly wanted to take down normal grocers, they could have purchased Kroger.
I think Whole Foods is a bit more in line with Amazon has already built with their grocery service, with a wide selection of high quality produce, pre-packaged foods, and ingredients. Meat prices are still a bit ridiculous there, but younger people tend to not mind paying a bit more for quality ingredients and food. Hence, avacado toast. The future has Americans spending more on groceries, not less. Lower priced chains will response to the move to automation in their own way, providing similar conveniences to compete.